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Taking a Sledgehammer to Our Climate

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Climate researcher Yarrow Axford taps unusual tools. The Northwestern University geologist relies on the exoskeletons of ancient insects and mud found at the bottom of Arctic lakes to trace chemical markers of climatic conditions throughout geologic history. She feels equally at home behind a microscope in a basement lab and camping beside Greenland’s massive ice sheet.

Axford said we have to go back three million years to find an atmospheric carbon dioxide concentration equal to today’s level. She talks about the “sledgehammer” human beings are swinging at natural systems by driving climate change.

Yarrow Axford (photo by Chris Bentley/MEDILL)Are we living in an unusual climate right now, geologically speaking? It depends on how far back you look. You can go back to times when carbon dioxide was as abundant in the atmosphere as it is today, but you have to go back at least three million years to find that, we think. Back then, the Arctic was incredibly warm — we don’t actually know if there was even sea ice. Beavers were living on Ellesmere Island, which is a harsh high Arctic tundra environment today, and my colleagues who do research there think summers were 15 to 20 degrees [Celsius] warmer there than they are today. Thanks to smaller ice sheets, global sea level was about 80 feet higher than today.

You work with aquatic invertebrates. What can these creatures tell you about Earth’s ancient climate?If you picture Lake Michigan today, anything you can imagine getting blown or washed into the lake ends up in the sediment at the bottom, and, if the environment is right, it gets preserved for thousands of years. So in a typical Arctic lake, far from direct human influences, the things you would find might be pollen grains, pine needles, bits of plants, insect remains.

Part of the fun is trying to figure out what’s useful to look at. Looking at the mix of species that were present turns out to be very useful. It’s one of the most intuitive ways to reconstruct the past environment. We also look at less intuitive things like oxygen isotopes in the chitin (the major component of an insect’s exoskeleton) of insect remains that we can trace back to the isotope composition in lake water, and ultimately all the way back up to climate.

What do lakes tell us about climate that other proxy measurements cannot? Part of what’s great about lakes is that they’re almost everywhere. We only have two big ice sheets on the planet: Greenland and Antarctica. So if you want to study ancient ice cores, you have to go to one of those two places. But what if you want to know about climate in Manitoba? Luckily there are lakes there.

What’s really interesting to me is that lakes preserve so many traces of past biology, so we can use lake sediments to look at how ecosystems respond to climate change.

Why study the Holocene — the youngest geological epoch, which began around 11,000 years ago? The Holocene was very unfashionable when I started working as a geologist. That’s partly because when you go back farther in time, the climate shifts that you get to look at are much larger, so those are exciting time periods.

I used to make the argument that we should study the Holocene to understand future climate change, because the size of the changes in the Holocene is more representative of what we could expect in the future. But I don’t think we can make that exact argument anymore.

What’s causing climate to change now is like a sledgehammer compared to tiny little taps through the Holocene. So we also need to go back to times when there was a sledgehammer acting on climate and see what happened. Both recent and older time periods have a lot to teach us.

How has studying natural history and working on such daunting scales of time and space changed your perspective on life? I think that that’s what got me interested in geology in the first place — the long-term perspective that it provided. As a freshman in college I started to get worried about things that were going on in the world, and then I discovered geology. I think it was comforting to realize that the planet has been around for billions of years.

It made humans seem very small in the scheme of things. But, ironically, I can’t say 15 years later, as a climate scientist, that we seem very small.

What’s next?I’m very excited about a project that I’ve been working on in Greenland. We’re trying to understand how sensitive the Greenland ice sheet is to temperature change. How the Greenland ice sheet will respond to warming over the next century is one of the biggest unanswered questions in climate science. We can try to model it, or we can go back in time and ask what the Greenland ice sheet did during past climate changes.

I’ll also be moving out of the Arctic to a very different environment, starting a new study in the Peruvian Andes. It will be challenging to study because it’s so new, and I expect the biology to be very different. But alpine ecosystems share heightened sensitivities with the arctic. For a long time we haven’t had very much information from the tropics at all. But there’s been an effort to start to fill in the gaps. I’d say it’s still a relatively little known part of the world in terms of climate history.

What would you most like to learn in your research career? On the one hand, I think it’s good for scientists to have a vision in terms of the questions they’d like to ask, but you don’t want to have a vision in terms of the conclusions you’d like to get to. For now I’ll just let the data keep leading me in new directions.

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is a publication by Science in Society Northwestern University's Office for science outreach and public engagement.